Monoolefins such as ethylene and propylene are often employed as monomers in preparing various polymers. Monoolefins may be produced by the thermal or catalytic cracking of refinery gas, ethane, propane, butane, and the like. Monoolefins produced in this manner usually contain acetylenes and diolefins that adversely affect polymer production. Thus, to form a polymer grade monoolefin, the amount of acetylenes and diolefins in a monoolefin stream recovered is typically reduced to form a polymer grade monoolefin.
One technique that has been used to reduce the amount of acetylenes and diolefins in a monoolefin stream involves hydrogenating the acetylenes and diolefins in the gas phase to monoolefins using catalysts comprising palladium and an inorganic support. Unfortunately, this total gas phase hydrogenation process does not entirely eliminate the acetylenes and the diolefins from the monoolefin stream. Further, in some applications total gas phase hydrogenation units produce green oil as a side-product by oligomerizing the acetylenes and diolefins. As used herein, green oil refers to molecules having 6 or more carbon atoms per molecule that undesirably foul the hydrogenation catalyst, causing the catalyst to deactivate. The deactivation of the hydrogenation catalyst results in the catalyst having a lower activity and selectivity to monoolefins. Consequently, the cycle time of total gas phase hydrogenation units are shortened by the presence of such green oil. Thus, the hydrogenation catalyst frequently requires regeneration, causing the operation costs of the hydrogenation process to be very high. Another technique that has been employed to reduce the acetylene content in a monoolefin stream has been the extractive distillation of the acetylenes. The extracted acetylenes have less commercial value than ethylene.
Therefore, a need exists for a less expensive process that improves removal of acetylenes and diolefins from a monoolefin stream without experiencing much fouling and that results in conversion of the acetylene to higher value chemicals. That is, it is desirable to extend the catalyst life and increase the selectivity of the hydrogenation catalyst to monoolefins such as ethylene.